Chapter - The Hydrosphere

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The Physical Environment: An Introduction to Physical Geography

CHAPTER 10: The Hydrosphere

A rain shaft pierces a tropical sunset as seen from Man-of-War Bay, Tobago, Caribbean

Sea . Most of the water evaporated from the ocean is directly returned by precipitation.

Courtesy NOAA

Water is a critical element that sustains life and drives a variety of environmental processes

acting within the Earth system. In this chapter we'll explore how water is cycled through and

its impact on the Earth system.



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The Hydrosphere Outline

The Hydrosphere

y Distribution of Water

The Hydrologic Cycle

y Evaporation and Condensation

y Interception and Infiltration

y Subsurface Water o Soil Water

o Groundwater

Groundwater and

Human Activities

y Surface water

The Water Balance

Review and Resources



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The Hydrosphere

The hydrosphere is often called the "water

sphere" as it includes all the earth's water found in streams, lakes, the soil,

groundwater, and in the air. Thehydrosphere interacts with, and is

influenced by, all the other earth spheres.

The water of the hydrosphere is distributed

among several different stores found in the

other spheres. Water is held in oceans,

lakes and streams at the surface of the

earth. Water is found in vapor, liquid and

solid states in the atmosphere. The

biosphere serves as an interface between

the spheres enabling water to move

between the hydrosphere, lithosphere and

atmosphere as is accomplished by planttranspiration. Thehydrologic cycle traces

the movement of water and energy between these various stores and spheres.

Figure TH.1 Earth Spheres/Systems

Distribution of water

The world's oceans contain 97% of the

water in the hydrosphere, most of which is

salt water. Ice caps, like that found

covering Antarctica, and glaciers thatoccupy high alpine locations, compose alittle less than 2% of all water found on

earth. Seemingly a small amount, thewater stored as ice in glaciers would have

a great impact on the environment if itwere to melt into a liquid. One fear is that

global warming will cause the melting andcollapse of large ice sheets resulting in sea

level rise. Rising sea levels could

devastate coastal cities, displace millions

of people, and wreak havoc on freshwater

systems and habitats.

Figure TH.2 The largest store of water is the

ocean which delivers water throughevaporation each day

Source: NOAA Photo LibraryUsed with permission

Water beneath the surface comprises the next largest store of water. Groundwater and soil

water together make up about .5% of all water (by volume). There is a difference between

ground water and soil water. S oil water is the water held in pore spaces between soil

particles. Soil pore spaces usually are partially void of water most of the time but fill with

water after a rain storm. Groundwater , on the other hand, is found where earth materials are



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saturated throughout the year. That is, the pore spaces are always occupied with water. Bothsoil and groundwater are very important sources of water. Soil water is available for plants to

extract and use. Groundwater is an important source of water for irrigation and drinkingwater supplies.

Above the surface water is found stored in streams, rivers and lakes. One might expect that

given the large rivers that flow across the earth and the huge numbers of lakes that this store

would be rather large. Instead, streams, rivers and lakes only comprise .02% of all water in

the earth system. In the atmosphere, only about .0001 % of the water in the hydrosphere is

found.

The Hydrologic Cycle

The hydrologic cycle, or water cycle, is the cycling of water through the earth system. Not

only is the hydrologic cycle a cycle of water, it is a cycle of energy as well. Over the next

several pages we'll trace water as it passes through the earth system and the energy that

accompanies it.

Figure TH.3 The Hydrologic cycle

Evaporation and Condensation

Evaporation is the phase change of liquid water into a vapor (gas). Evaporation is an

important means of transferring energy between the surface and the air above. The energy

used to evaporate water is called "latent energy". Latent energy is "locked up" in the water molecule when water undergoes the phase change from a liquid to a gas. Eighty-eight percent

of all water entering the atmosphere originates from the ocean between 60o

north and 60o

south latitude. Most of the water evaporated from the ocean returns directly back to the



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ocean. Some water is transported over land before it is precipitated out. When water vapor condenses back into a liquid it releases latent heat, which is converted into sensible heat

warming the surrounding air. The warming of the surrounding air fuels uplift to help promoteadiabatic cooling and further condensation. As droplets of water coalesce into larger droplets

they attain a size big enough to fall towards the earth as precipitation. Located high in thetroposphere, rain drops possess a high degree of potential energy that is converted into kinetic

energy once they begin to fall toward the surface. Impacting the surface they convert thiskinetic energy into work done on the surface (erosion for example).

Interception and Infiltration

As water reaches the surface in various forms of precipitation, it is intercepted by plants or

falls directly to the surface. Precipitation that collects on the leaves or stems of plants isknown as interception. The amount of water intercepted by a plant largely depends on plant

form. Water is held on the leaf surface until it either drips off asthrough fall or trickles downthe leaf stem finally reaching the ground as stem flow. Interception of falling rain buffers the

surface against erosion. Coniferous trees tend to intercept more water than deciduous trees on

an annual basis because deciduous trees drop their leaves for a period of time.

Figure TH.4 Droplets of water intercepted by

tree leaf.

( Source: M. Marzot FAO. Used with permission)

Upon reaching the ground, some water infiltrates into the soil, possibly percolating down to

the groundwater zone or it may run across the surface as runoff . Infiltrationrefers to water

that penetrates into the surface of soil. Infiltration is controlled by soil texture, soil structure,

vegetation and soil moisture status. High infiltration rates occur in dry soils, with infiltration

slowing as the soil becomes wet. Coarse textured soils with large well-connected pore spaces

tend to have higher infiltration rates than fine textured soils. However, coarse textured soils

fill more quickly than fine textured soils due to a smaller amount of total pore space in a unit

volume of soil. Runoff is generated quicker than one might have with a finer textured soil.

Vegetation also affects infiltration. For instance, infiltration is higher for soils under forest

vegetation than bare soils. Tree roots loosen and provide conduits through which water can

enter the soil. Foliage and surface litter reduce the impact of falling rain keeping soil passages

from becoming sealed.

Subsurface water

Groundwater and soil water together comprise approximately .5% of all water in the

hydrosphere. Water beneath the surface can essentially be divided into two zones (Figure

TH.5), the unsaturated zone ( also known as the "zone of aeration") which includes soil water



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zone, and zone of saturation which includes ground water. Air and water occupies the porespaces between earth materials in the zone of aeration. Sometimes, especially during times of

high rainfall, these pore spaces are filled with water. The water tabledivides the zone of aeration from the zone of saturation. The height of the water table will fluctuate with

precipitation, increasing in elevation during wet periods and decreasing during dry. Note howthe water table intersects the level of the stream surface in Figure TH.5. Seepage of

groundwater into a stream provides a base flow of water for perennial streams.

Figure TH.5 Zones of water beneath the surface

Courtesy USGS (Source)

Soil Water

Soil water is held in the pore spaces between particles of soil. Soil water is the water that is

immediately available to plants. Soil water can be further sub-divided into three categories, 1)

hygroscopic water, 2) capillary water, and 3) gravitational water. H ygroscopic water is found

as a microscopic film of water surrounding soil particles. This water is tightly bound to a soil

particle by molecular attraction so powerful that it cannot be removed by natural forces.

Hygroscopic water is bound to soil particles by adhesive forces that exceed 31 bars and may

be as great as 10,000 bars (Recall that sea level pressure is equal to 1013.2 millibars which is

just about 1 bar!). Capillary water is held by cohesive forces between the films of

hygroscopic water. The binding pressure for capillary water is much less than hygroscopic

water. This water can be removed by air drying or by plant absorption, but cannot be

removed by gravity. Plants extract this water through their roots until the soil capillary force(force holding water to the particle) is equal to the extractive force of the plant root. At this

point the plant cannot pull water from the plant-rooting zone and it wilts (called the wilting

point ). Gravity water is water moved through the soil by the force of gravity. The amount of

water held in the soil after excess water has drained is called the field capacity of the soil.The amount of water in the soil is controlled by the soil texture. Soils dominated by clay-

sized particles have more total pore space in a unit volume than soils dominated by sand. As aresult, fine grained soils have higher field capacities than coarse-grained soils.



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Table TH.1 Representative Porosity Ranges for Sedimentary Materials

Material Porosity (%)Soils

Clay

Silt

Uniform sand

Gravel

Sandstone

Shale

50 - 60

45 - 55

40 - 50

30 - 40

30 - 40

10 - 20

1 - 10

From Ground Water Hydrology by D. K. Todd, 1959, p. 16, John Wiley &

Sons, Inc.

The diagram below shows the relationship between soil texture, wilting point, field capacity,

and available water. The difference between the wilting point and the field capacity is

the available water .

Figure TH.6 Relationship Between Available Water and Soil Texture

Note that the smallest amount of available water is associated with the coarsest soil texture,

sand. The amount of available water increases toward the center of the graph where soils witha mixture of different sized particles (loamy soils) are found. The available water then drops

off toward the fine textured soils on the right. How does one explain the relationship betweenavailable water and soil texture? Coarse soil does not have much available water because it

doesn't hold much water to begin with. At the other end of the spectrum, low available water



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in fine soils is due to strong bond between soil particles and water. Plants have a harder time pulling the water away from the soil particle under these conditions.

Ground water

Ground water occupies the zone of saturation. As depicted in the hydrologic cycle diagram,ground water moves downward through the soil by percolation and then toward a stream

channel or large body of water as seepage. The water table separates the zone of saturation

from the zone of aeration. The water table fluctuates with moisture conditions, during wet

times the water table will rise as more pore spaces are occupied with water. Ground water is

found inaquifers, bodies of earth material that have the ability to hold and transmit water.

Aquifers can be either unconfined or confined. Unconfined (open) aquifers are "connected" to

the surface above. Confined (closed) aquifers are sandwiched in between dense impermeablelayers of earth material called an aquiclude. Ground water is replenished by percolation of

water from the zone of aeration downward to the zone of saturation, or in the recharge zoneof a confined aquifer. The recharge zone is where the confined aquifer is exposed at the

surface and water can enter it.

Figure TH.7 Unconfined and confined aquifers


Aquifers replenish their supply of water very slowly. The rate of ground water flow depends

on the permeability of the aquifer and the hydraulic gradient. Permeability is affected by the

size and connectivity of pore spaces. Larger, better connected pore spaces creates highly

permeable earth material. The hydraulic gradient is the difference in elevation between two points on the water table divided by the horizontal distance between them. The rate of ground

water flow is expressed by the equation:

Ground water flow rate = permeability X hydraulic gradient



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Ground water flow rates are usually quite slow. Average ground water flow rate of 15 m per

day is common. Highly permeable materials like gravels can have flow velocities of 125 m

per day.

TH.8 Ground water movement


Ground water in an aquifer is under pressure called hydrostatic pressure. Hydrostatic

pressure in a confined aquifer pushes water upward when a well is drilled into the aquifer.

The height to which the water rises is called the peizometeric surface. If the hydrostatic

pressure is great enough to push the peizometeric surface above the elevation of the surface,

water readily flows out as an artesian well.

Figure TH. 9 Pivot

irrigation on a barley field

in Saudi Arabia. The barley

is used as fodder for milk

cows.(Source: F. Mattioli,

FAO Used with permission)



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Ground water and Human Activities

Ground water is an important source of water for human activities such as agriculture and

domestic drinking water. In 2000, 68% of fresh ground water was used for irrigation while

another 19% was extracted for public-supply purposes, mostly drinking water. For those whosupply their own water for domestic use, over 98 percent is from ground water.

Figure TH. 10 United States groundwater withdrawals (2000) Courtesy USGs

In dry regions and in places where soils are highly permeable, agriculture uses large amounts

of groundwater for irrigation. The high rate of water use for agriculture has fueled tensions between urban and rural interests. ADec. 4, 1998 "Talk of the Nation - Science Friday"

segment, "San Antonio: Water Rights", discusses the competing demands of urban and rural populations using the Edwards Aquifer in south Texas. (RealAudio).

Figure TH.11 Outcrop of the Ogallala (sandstone)

Formation, the main water-bearing unit of the High

Plains aquifer, near the Canadian River inTexas. Courtesy USGS.



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However, the rate of ground water removal by humans sometimes exceeds the recharge of the

aquifer. Such is the case of the High Plains Aquifer of Arizona, Colorado, New Mexico, and

Utah. Over pumping of ground water reserves can lead to compaction and degradation of the

aquifer. As the water is removed, the aquifer often collapses causing the surface to subside.

Aquifer compaction reduces pore space, making recharge more difficult. One of the most

striking examples is that which has occurs in the San Joaquin Valley of California.

Figure TH.12 Land subsidence in

California

Joe Poland, US GS scientist shows subsidence from 1925 and 1977 10 miles

southwest of Mendota, CA. Sign reads "San Joaquin Valley California, BM S661,

Subsidence 9M, 1925-1977" From US GS

Professional Paper 1401-A, " Ground water

in the Central Valley, California- A summary

report"

Photo by Dick Ireland, US GS, 1977

Surface Water

Once precipitation reaches the surface, water can infiltrate into the soil or move across the

surface as runoff . Surface runoff generally occurs when the rainfall intensity exceeds the rate

of infiltration, or if the soil is at its water holding capacity. Infiltration and water holding

capacity are both a function of soil texture and structure. Soil composed of a high percentage

of sand allows water to infiltrate through it quite rapidly because it has large, well-connected

pore spaces. Soils dominated by clay have low infiltration rates due to their smaller sized

pore spaces. However, there's actually less total pore space in a unit volume of coarse, sandy



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soil than that of soil composed mostly of clay. As a result, sandy soils fill rapidly andcommonly generate runoff sooner than clay soils.

Figure TH.13 Meandering stream

(Source: Geological Survey of Canada Used withPermission)

If the rainfall intensity exceeds the infiltration capacity of the soil, or if the soil has reached

its field capacity, surface runoff occurs. Water runs across the surface as either confined or

unconfined flow. U nconfined flowmoves across the surface in broad sheets of water often

creating sheet erosion. Confined flow refers to water confined to channels. Stream flow is a

form of confined flow.

Water that runs along the surface may become trapped in depressions and held as depression

storage. Here water may either evaporate back into the air, infiltrate into the ground or, spill

out of the depression as it fills.

Stream flow is measured in a variety of ways, one of which is stream discharge. S tream

discharge is the volume of water passing through a particular cross-section of a stream in aunit of time. Stream discharge is measure in cubic feet per second or cubic meters per second.

The "normal" or base flow a stream is provided by seepage of groundwater into the streamchannel. This seepage is what keeps perennial flowing streams going all year. When

precipitation from a storm occurs, the stream discharge increases as water is added to thestream, either from direct precipitation into the channel or runoff.

Figure TH.14 A stream hydrograph



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A stream hydrograph (right) illustrates the relationship between discharge and runoff. The

blue bar graph is precipitation and the line graph represents discharge. As time passes

(measured along the x-axis) discharge increases as precipitation falls. Notice that the time of

peak precipitation occurs before peak discharge. This is because it takes time for the water to

flow across the surface and enter the stream.

The size, shape, land use, vegetation, and geology of the watershed all determine runoff andthe shape of the discharge graph.

Figure TH.15 Total United States Water

Withdrawals, 2000Courtesy USGS

Surface water is an important source of water supplies, especially in the southwestern United

States. Ever increasing population and development is straining regional water resources. As

the Colorado River winds its way through the desert it loses half its flow to evaporation and

the rest to irrigation and municipal water supplies before reaching the Gulf of California.Recent drought and warmer temperatures predicted in the future will put a greater demand onthis precious commodity.

The Water Balance

The water balance is an accounting of the inputs and outputs of water. The water balance of a

place, whether it be an agricultural field, watershed, or continent, can be determined bycalculating the input, output, and storage changes of water at the Earth's surface. The major

input of water is from precipitation and output is evapotranspiration. The geographer C. W.Thornthwaite (1899-1963) pioneered the water balance approach to water resource analysis.

He and his team used the water-balance methodology to assess water needs for irrigation and

other water-related issues.

The Water Balance

To understand water-balance concept, we need to start with its various components:



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P recipitation (P). Precipitation in the form of rain, snow, sleet, hail, etc. makes up the primarily supply of water to the surface. In some very dry locations, water can be supplied by

dew and fog.

Actual evapotranspiration (AE). Evaporation is the phase change from a liquid to a gas

releasing water from a wet surface into the air above. Similarly, transpiration is represents a

phase change when water is released into the air by plants. Evapotranspiration is the

combined transfer of water into the air by evaporation and transpiration. Actual

evapotranspiration is the amount of water delivered to the air from these two processes.

Actual evapotranspiration is an output of water that is dependent on moisture availability,

temperature and humidity. Think of actual evapotranspiration as "water use", that is, water

that is actually evaporating and transpiring given the environmental conditions of a place.Actual evapotranspiration increases as temperature increases, so long as there is water

to evaporate and for plants to transpire. The amount of evapotranspiration also depends onhow much water is available, which depends on the field capacity of soils. In other words, if

there is no water, no evaporation or transpiration can occur.

Figure TH.16 The soil water

balance (After Strahler &

Strahler, 2006)

P otential evapotranspiration(PE). The environmental conditions at a place create a demand

for water. Especially in the case for plants, as as energy input increases, so does the demandfor water to maintain life processes. If this demand is not met, serious consequences can

occur. If the demand for water far exceeds that which is actual present, dry soil moisture

conditions prevail. Natural ecosystems have adapted to the demands placed on water.

Potential evapotranspiration is the amount of water that would be evaporated under anoptimal set of conditions, among which is an unlimited supply of water. Think of potential

evapotranspiration of "water need". In other words, it would be the water needed for

evaporation and transpiration given the local environmental conditions. One of the most

important factors that determines water demand is solar radiation. As energy input increases

the demand for water, especially from plants increases. Regardless if there is, or isn't, any

water in the soil, a plant still demands water. If it doesn't have access to water, the plant will

likely wither and die.



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S oil Moisture S torage (ST). Soil moisture storage refers to the amount of water held in thesoil at any particular time. The amount of water in the soil depends soil properties like soil

texture and organic matter content. The maximum amount of water the soil can hold is calledthe field capacity. Fine grain soils have larger field capacities than coarse grain (sandy) soils.

Thus, more water is available for actual evapotranspiration from fine soils than coarse soils.The upper limit of soil moisture storage is the field capacity, the lower limit is 0 when the soil

has dried out.

Change in S oil Moisture S torage (ST). The change in soil moisture storage is the amount

of water that is being added to or removed from what is stored. The change in soil moisture

storage falls between 0 and the field capacity.

Deficit ( D) A soil moisture deficit occurs when the demand for water exceeds that which is

actually available . In other words, deficits occur when potential evapotranspiration exceedsactual evapotranspiration (PE>AE). Recalling that PE is water demand and AE is actual

water use (which depends on how much water is really available), if we demand more thanwe have available we will experience a deficit. But, deficits only occur when the soil is

completely dried out . That is, soil moisture storage (ST) must be 0. By knowing the amountof deficit, one can determine how much water is needed from irrigation sources.

S urplus (S) Surplus water occurs when P exceeds PE and the soil is at its field capacity

(saturated). That is, we have more water than we actually need to use given the

environmental conditions at a place. The surplus water cannot be added to the soil becausethe soil is at its field capacity so it runs off the surface. Surplus runoff often ends up in nearby

streams causing stream discharge to increase. A knowledge of surplus runoff can helpforecast potential flooding of nearby streams.

Computing a Soil - Moisture Budget

The best way to understand how the water balance works is to actually calculate a soil water budget. We'll use Rockford, Illinois which is located in the humid continental climate of

northern Illinois. Rockford lies on the northern edge of the prairie and mixes with deciduousforest. This vegetation has been nearly completely replaced with agriculture. A knowledge of

soil moisture status is important to the agricultural economy of this region that produces

mostly corn and soy beans.

To work through the budget, we'll take each month (column) one at a time. It's important to

work column by column as we're assessing the moisture status in a given month and one

month's value may be determined by what happened in the previous month.

Table TH.2 Water Budget - Rockford, IL

Field Capacity = 90 mm

J F M A M J J A S O N D Year

P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45



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ST 0 0 0 0 0 17 57 16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

Soil Moisture Recharge

Table TH.3 Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mm


P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

We'll start the budget process at the end of the dry season when precipitation begins to

replenish the soil moisture, called soil moisture recharge, in September. At the beginning of

the month the soil is considered dry as the storage in August is equal to zero. During

September, 86 mm of water falls on the surface as precipitation. Potential evapotranspiration

requires 85 mm. Precipitation therefore satisfies the need for water with one millimeter of

water left over (P-PE=1). The excess one millimeter of water is put into storage (ST=1) bringing the amount in storage to one millimeter (August ST =0 so 0 plus the one millimeter

in September equals one millimeter). Actual evapotranspiration is equal to potential

evapotranspiration as September is a wet month (P>PE). There is no deficit during this monthas the soil now has some water in it and no surplus as it has not reached its water holding

capacity.

During the month of October, precipitation far exceeds potential evapotranspiration (P-PE=29). All of the excess water is added to the existing soil moisture (ST (September) + 29

mm = 30 mm). Being a wet month, AE is again equal to PE.

Calculating the budget for November is very similar to that of September and October. Thedifference between P and PE is all allocated to storage (ST now equal to 78 mm) and AE is

equal to PE.

Soil Moisture Surplus

During December, Rockford is deep in the grip of winter. Potential evapotranspiration has

dropped to zero as plants have gone into a dormant period thus reducing their need for water

and cold temperatures inhibit evaporation. Notice that P-PE is equal to 45 but not all is placed



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into storage. Why? At the end of November the soil is within 12 mm of being at its fieldcapacity. Therefore, only 12 millimeters of the 45 available is put in the soil and the

remainder runs off as surplus (S=33).

Table TH.4 Soil Moisture Surplus - Rockford, IL



P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

Given that the soil has reached its field capacity in December, any excess water that falls on

the surface in January will likely generate surplus runoff. According to the water budget table

this is indeed true. Note that P-PE is 50 mm and ST is 0 mm. What this indicates is that we

cannot change the amount in storage as the soil is at its capacity to hold water. As a result the

amount is storage (ST) remains at 90 mm. Being a wet month (P>PE) actual

evapotranspiration is equal to potential evapotranspiration. Note that all excess water (P-PE)

shows up as surplus (S=50 mm).

Similar conditions occur for the months of February, March, April, and May. These are all

wet months and the soil remains at its field capacity so all excess water becomes surplus.

Note too that the values of PE are increasing through these months. This indicates that plants

are springing to life and transpiring water. Evaporation is also increasing as insolation and air

temperatures are increasing. Notice how the difference between precipitation and potential

evapotranspiration decreases through these months. As the demand on water increases,

precipitation is having a harder time satisfying it. As a result, there is a smaller amount of

surplus water for the month.

Surplus runoff can increase stream discharge to the point where flooding occurs. The flood

duration period lasts from December to May (6 months), with the most intense flooding islikely to occur in March when surplus is the highest (61 mm).



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Soil Moisture Utilization

Table TH.5 Soil Moisture Utilization - Rockford, IL



P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

By the time June rolls around, temperatures have increased to the point where evaporation is

proceeding quite rapidly and plants are requiring more water to keep them healthy. As potential evapotranspiration is approaching its maximum value during these warmer months,

precipitation is falling off.During June P-PE is -17 mm. What this means is precipitation nolonger is able to meet the demands of potential evapotranspiration. In order to meet their

needs, plants must extract water that is stored in the soil from the previous months. This isshown in the table by a value of 17 in the cell for ST (change in soil storage). Once the 17

m is taken out of storage (ST) it reduces its value to 73.

The month of June is considered a dry month (P<PE) so AE is equal to precipitation plus the

absolute value of ST (P + |ST|). When we complete this calculation (106 mm + 17 mm =

123 mm) we see that AE is equal to PE. What this means is precipitation and what was

extracted from storage was able to meet the needs demanded by potential evapotranspiration.

Note that there is no surplus in June as the soil moisture storage has dropped below its fieldcapacity. There is still no deficit as water remains in storage. The calculations for July issimilar to June, just different values. Note that by the time July ends, water held in storage is

down to a mere 16 mm.



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Soil Moisture Deficit

Table TH.6 Soil Moisture - Rockford, IL



P 50 49 66 78 100 106 88 84 86 73 56 45 881

PE 0 0 5 40 84 123 145 126 85 44 8 0 531

P-PE 50 49 61 38 16 -17 -57 -42 1 29 48 45

ST 0 0 0 0 0 -17 -57 -16 1 29 48 12

ST 90 90 90 90 90 73 16 0 1 30 78 90

AE 0 0 5 40 84 123 145 100 85 44 8 0 634

D 0 0 0 0 0 0 0 26 0 0 0 0 26

S 50 49 61 38 16 0 0 0 0 0 0 33 258

August, like June and July, is a dry month. Potential evapotranspiration still exceeds

precipitation and the difference is a -42 mm. Up until this month there has been enough water from precipitation and what is in storage to meet the demands of potential evapotranspiration.

However, August begins with only 16 mm of water in storage (ST of July). Thus we'll only be able to extract 16 mm of the 42 mm of water needed to meet the demands of potential

evapotranspiration So, of the 42 mm of water we would need (P-PE) to extract from the soil.In so doing, the amount in storage (ST) falls to zero and the soil is dried out. What happens to

the remaining 26 mm of the original P-PE of 42? The unmet need for water shows up as soilmoisture deficit. In other words, we have not been able to meet our need for water from both

precipitation and what we can extract from storage. AE is therefore equal to 100 mm (84 mm

of precipitation plus 16 mm of ST).

So what is a farmer to do if their crops cannot obtain needed water from precipitation or soil

moisture storage....they irrigate. Irrigation water usually is pumped from groundwater supplies held in aquifers deep below the surface or from nearby streams (if stream flow issufficient to provide needed water). The amount of irrigation water required is the amount of

the soil moisture deficit.



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Soil Moisture Seasons

Four soil moisture seasons can be defined by the soil moisture conditions.

Figure TH.17 Soil

Moisture Seasons for

Rockford, Illinois

Recharge

The recharge season is a time when water is added to soil moisture storage (+ST). The

recharge period occurs when precipitation exceeds potential evapotranspiration but the soil

has yet to reach its field capacity.

Surplus

The surplus season occurs when precipitation exceeds potential evapotranspiration and the

soil has reached its field capacity. Any additional water applied to the soil runs off. If this

water runs off into nearby streams and rivers it could cause flooding. Thus, the intensity

(amount) and duration (length of season) of surplus can be used to predict the severity of

potential flooding.

Utilization

The utilization season is a time when water is withdrawn from soil moisture storage (-ST).

The utilization period occurs when potential evapotranspiration exceeds precipitation but soilstorage has yet to reach 0 (dry soil).

Deficit

The deficit season occurs when occurs when potential evapotranspiration exceeds

precipitation and soil storage has reached 0. This is a time when there is essentially no water

for plants. Farmers then tap ground water reserves or water in nearby streams and lakes to



The Hydrosphere 259


irrigate their crops. Thus, the intensity (amount) and duration (length of season) of deficit can be used to predict the need for irrigation water.

Whether a place experiences all four seasons depends on the climate and soil properties. Wet

climate and those places with soils having high field capacities are less likely to experience a

deficit period. Likewise the duration and intensity of any season will be determined by the

climate and soil properties. Given equal amounts of precipitation, coarse textured soils will

generate runoff faster than fine textured soils and may experience more intense surplus



The Hydrosphere 260

Review

Use the links below to review and assess your learning. Start with the "Important Terms and

Concepts" to ensure you know the terminology related to the topic of the chapter and

concepts discussed. Move on to the "Review Questions" to answer critical thinking questions

about concepts and processes discussed in the chapter. Finally, test your overallunderstanding by taking the "Self-assessment quiz".

y Important Terms and Concepts

y Review Questions

y Self-assessment test

Additional Resources

Use these resources to further explore the world of geography

Multimedia

"The Desert Springs of Mexico's Cuatro Cienegas", (8:56)

Readings

"Defining Drought" National Drought Mitigation Center, University of Nebraska -

Lincoln

Web Sites

Water Resources of the United States - United States Geological Survey

National Oceanic and Atmospheric Administration (NOAA)

Documents

Chapter - The Hydrosphere